The RegisterDefaultFilterExtList method registers a list of file name extensions for which you want to use the default IFilter, when no other IFilter registered for the file name extension successfully handles the document.
HRESULT RegisterDefaultFilterExtList ( WCHAR const *pwcsOverride, WCHAR const *pwszzExtList, WCHAR const *pwcsMachineName);
[in] A pointer to a WCHAR that contains an override string, such as the application name.
[in] A pointer to a WCHAR buffer that contains one or more null-terminated file name extension-wide strings. The last extension is terminated with two null characters.
[in] Not used. Value should be 0.
For a list of errors returned by SharePoint Portal Server IFilter registration objects, see Error Messages.
This method stores the registry settings that specify the file name extensions that explicitly use the default IFilter if no other IFilter registered for that file type is successful.
If the pwcsOverride string is not null, the method registers the file name extensions using that override string. The pwcsMachineName string indicates the computer name on which the settings are made.